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Title Influences of mesoscale anticyclonic eddies on the zooplankton community south of the western Aleutian Islandsduring the summer of 2010

Author(s) Saito, Rui; Yamaguchi, Atsushi; Yasuda, Ichiro; Ueno, Hiromichi; Ishiyama, Hiromu; Onishi, Hiroji; Imai, Ichiro

Citation Journal of plankton research, 36(1), 117-128https://doi.org/10.1093/plankt/fbt087

Issue Date 2014-01

Doc URL http://hdl.handle.net/2115/57138

RightsThis is a pre-copy-editing, author-produced PDF of an article accepted for publication in Journal of Plankton Researchfollowing peer review. The definitive publisher-authenticated version J. Plankton Res. (January/February 2014) 36 (1):117-128. is available online at:http://plankt.oxfordjournals.org/cgi/content/full/fbt087?ijkey=XsssmgLRwbO2kGH&keytype=ref

Type article (author version)

File Information Saito_et_al_2014_J_Plankton_Res_2.pdf

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

1

Influences of mesoscale anticyclonic eddies on the zooplankton community south of the 1

western Aleutian Islands during the summer of 20102

Rui Saito1*, Atsushi Yamaguchi1, Ichiro Yasuda2, Hiromichi Ueno3,4, Hiromu Ishiyama4, Hiroji 3

Onishi3, Ichiro Imai14

1Plankton Laboratory, Graduate School of Fisheries Sciences, Hokkaido University, 311 5

Minato-cho, Hakodate, Hokkaido, 0418611, Japan6

2Fisheries Environmental Oceanography Laboratory, Atmosphere and Ocean Research Institute, 7

The University of Tokyo, 511 Kashiwanoha, Kashiwa, Chiba, 2778564, Japan8

3Physical Environmental Science Laboratory, Faculty of Fisheries Sciences, Hokkaido University, 9

311 Minato-cho, Hakodate, Hokkaido, 0418611, Japan10

4Course in Marine Biogeochemistry and Physics, Graduate School of Environmental Science, 11

Hokkaido University, North 10, West 5, Kita-ku, Sapporo, Hokkaido, 0600810, Japan12

*Corresponding author: Rui Saito. E-mail address: rsaito@aori.u-tokyo.ac.jp13

Present address: Fisheries Environmental Oceanography Laboratory, Atmosphere and Ocean 14

Research Institute, The University of Tokyo, 511 Kashiwanoha, Kashiwa, Chiba, 2778564, 15

Japan16

Telephone number: 81471366244; Fax number: 8147136624717

“This is a pre-copy-editing, author-produced PDF of an article accepted for publication in Journal 18

of Plankton Research following peer review. The definitive publisher-authenticated version J. 19

Plankton Res. (January/February 2014) 36 (1): 117-128. is available online at: 20

http://plankt.oxfordjournals.org/cgi/content/full/fbt087?ijkey=XsssmgLRwbO2kGH&keytype=ref ”21

2

Abstract22

Mesoscale anticyclonic eddies have been observed south of the Aleutian Islands. Eddies 23

farther east, in the Gulf of Alaska, are known to transport coastal water and coastal zooplankton to 24

offshore open ocean. The impacts of mesoscale anticyclonic eddies formed south of the western 25

Aleutian Islands (Aleutian eddies) on the zooplankton community are not fully understood. In the 26

present study, we describe zooplankton population structures within an Aleutian eddy and outside 27

the eddy during July 2010. Based on the sea level anomaly, the Aleutian eddy was formed south 28

of Attu Island (17254E) in February 2010, and it moved southeastward in the next five months. 29

Large oceanic copepods, Neocalanus cristatus, Eucalanus bungii and Metridia pacifica were more 30

abundant inside the eddy than the outside. Inside the eddy, the life stage distribution of N. 31

cristatus was advanced than that outside, and Neocalanus spp. had accumulated more lipids. 32

These conditions probably reflect the greater primary production in the eddy, production enhanced 33

by nutrients advected into the eddy. The Aleutian eddy contained mostly oceanic copepods 34

because it was formed in the offshore water and/or eddy-eddy interaction occurred after its 35

formation. The sufficient food condition in the eddy presumably induced higher growth and 36

survival rates of these oceanic copepods, resulting in the greater abundance, advanced development37

stages and greater lipid accumulation.38

Keywords: mesoscale anticyclonic eddies; Aleutian eddies; zooplankton; calanoid copepods39

3

Introduction40

The Alaskan Stream is the northern boundary current of the North Pacific Subarctic Gyre, 41

flowing westward along the shelf break and the Aleutian Trench, south of the Alaska Peninsula and 42

the Aleutian Islands (Favorite, 1967; Ohtani et al., 1997; Reed and Stabeno, 1999). The Alaskan 43

Stream connects the Alaskan Gyre, the Bering Sea Gyre and the Western Subarctic Gyre (Onishi, 44

2001).45

Along the coasts of the Gulf of Alaska and the Aleutian Islands, several types of mesoscale 46

anticyclonic eddies are known to be formed (Fig. 1A). Haida eddies appear west of Haida Gwaii 47

(formerly called the Queen Charlotte Islands at 5300N, 13200W) and Alexander Archipelago48

(5640N, 13405W), and propagate northwestward into the central Gulf of Alaska (Crawford et al., 49

2000; Crawford, 2002, 2005). Sitka eddies form off Sitka, Alaska (5703N, 13519W), and 50

propagate northwestward (Crawford et al., 2000; Rovegno et al., 2009). Yakutat eddies appear in 51

the northern Gulf of Alaska, off Yakutat, Alaska (5945N, 14042W) and move westward along 52

the Alaskan Stream (Ladd et al., 2005, 2007; Janout et al., 2009). Kenai eddies form south of the 53

Kenai Peninsula between 143W and 160W, and propagate southwestward along the Alaskan 54

Stream (Rovegno et al., 2009; Lippiatt et al., 2011; Ueno et al., 2012). These eddies do not cross 55

the 180 meridian (Ueno et al., 2009). Anticyclonic eddies called Alaskan Stream eddies appear56

in the Alaskan Stream region between 157W and 169W, south of the Alaska Peninsula and 57

Aleutian Islands (Ueno et al., 2009). The Alaskan Stream eddies usually move westward for 15 58

years and sometimes cross the 180 meridian and reach the Western Subarctic Gyre. Mesoscale 59

anticyclonic eddies also form in the western Alaskan Stream region (Rogachev et al., 2007; 60

Rogachev and Shlyk, 2009). These eddies form in the region between the 180 meridian and 61

Near Strait (about 170E) and are called Aleutian eddies. Many of the Aleutian eddies move 62

southwestward, and reach the Western Subarctic Gyre.63

Mesoscale anticyclonic eddies observed in the Alaskan Stream and the Alaska Current64

regions (Fig. 1A) are thought transport significant mass of coastal water to the offshore open ocean. 65

4

For example, eddies in the Gulf of Alaska bring coastal water (which is warm, has a low-salinity 66

and is rich in nutrient and iron) to the offshore oceanic region (Crawford, 2005; Lippiatt et al., 67

2011; Brown et al., 2012). Satellite images show that these eddies are high in surface chlorophyll 68

and primary production from spring through summer (Crawford et al., 2005, 2007). Alaskan 69

Stream eddies are also high in chlorophyll and hence primary production (Ueno et al., 2010). A 70

recent study of a Haida eddy showed that the phytoplankton assemblage in the eddy was dominated 71

by diatoms, but as the eddy drifted away from the coast, the amount of diatoms significantly 72

decreased (Peterson and Harrison, 2012). Phytoplankton diversity inside that eddy greater than in 73

waters outside of it in autumn during the eddy’s later evolution (Peterson et al., 2011). These 74

mesoscale anticyclonic eddies are thought to influence strongly the density of phytoplankton in the 75

central subarctic North Pacific (Ueno et al., 2010).76

Mesoscale anticyclonic eddies with high primary production in the Alaskan Stream region 77

are thought to influence the zooplankton, which could nourish higher trophic levels and enhance 78

fish production. The zooplankton community in Haida eddies has been reported to have a mixed 79

community of coastal and oceanic species at the point of formation, and then the abundance of 80

coastal species gradually decreased over time (Mackas and Galbraith, 2002; Mackas et al., 2005). 81

Analysis using a continuous plankton recorder (CPR) of mesoscale anticyclonic eddies in the Gulf 82

of Alaska also showed that coastal calanoid copepods are abundant inside them transporting these 83

coastal species offshore (Batten and Crawford, 2005). Thus, the impacts of mesoscale 84

anticyclonic eddies in the Gulf of Alaska on zooplankton communities have gradually come to 85

understood. However, the influences of Aleutian eddies south of the western Aleutian Islands on 86

their entrained zooplankton communities are not fully understood.87

In the present study, we compared vertical profiles of hydrography and the zooplankton 88

communities between waters inside and outside of an Aleutian eddy for the first time. Analyses of89

population structure and lipid accumulation of large oceanic calanoid copepods demonstrate the 90

possible impacts of that eddy on the growth and nutritive condition of the copepods.91

5

Method92

Field study93

Our field study was conducted at seven stations along 5115N from 17121E to 17438E 94

and at four stations along 5040N from 17624E to 17844E on board T/S Oshoro-Maru of the 95

Faculty of Fisheries, Hokkaido University, during 78 July 2010 (Fig. 1B). At each station, 96

temperature, salinity and fluorescence were measured with a CTD (Sea-Bird Electronics, Inc., U. S. 97

A., CTD-SBE 9plus). At some stations, only temperature and salinity were measured by an 98

XCTD (Tsurumi Seiki Co., Ltd., Japan). These hydrographic data have been published 99

elsewhere (Hokkaido University, 2011).100

Zooplankton samples were collected by vertical tows from 150 m to the surface using a 45 101

cm mouth diameter, 100 m mesh size NORPAC net (Motoda, 1957) equipped with a flowmeter 102

(Rigosha Co., Ltd., Japan). The net towing speed was 1 m s1. During each sampling, the wire 103

angle was measured using a protractor, and the wire length was extended until the net reached the 104

desired depth. Samples were immediately preserved in 5% formalin-seawater buffered with 105

sodium tetraborate. The volume of water filtered was calculated from the flowmeter reading.106

Data and sample analyses107

To evaluate the position of mesoscale anticyclonic eddies, delayed-time data of sea level 108

anomaly (SLA) in the period from the approximate date of eddy formation (6 January 2010) to the 109

date of field sampling (7 July 2010) were downloaded from AVISO (Collecte Localisation 110

Satellites, France; http://www.aviso.oceanobs.com; SSALTO/DUACS, 2012). The spatial 111

resolution was 1/4 1/4. The SLA data at seven-day intervals was used to track eddies. 112

During summer, seawater expands due to the increase in water temperature, thus using raw SLA 113

data, the sea-level anomalies in the whole region tend to be positive in summer and negative in 114

winter (Ueno et al., 2012). Accordingly, the weekly spatial mean state of the subarctic North 115

6

Pacific north of 45N, except for the marginal seas, was removed from each weekly map of SLA to 116

compensate for seasonal steric effects (Ueno et al., 2009, 2010, 2012). Eddies were tracked using 117

the Okubo-Weiss parameter: W (Okubo, 1970; Weiss, 1991) calculated from the SLA data118

assuming geostrophy. In this analysis, we defined an area with W < 2 1012 s2 as an eddy area 119

(Chelton et al., 2007). The eddy area and the position of the eddy centre were analyzed, and the 120

eddies were tracked in the same manner as by Henson and Thomas (2008); Inatsu (2009) and Ueno 121

et al. (2012). The positions of eddy centres estimated from SLA data may have errors > 50 km 122

due to data resolution and eddy propagation (Ladd et al., 2005, 2007).123

In the land laboratory of Hokkaido University, each zooplankton sample was mixed well, 124

and a 1/10 subsample was taken using a large bore pipette. The subsample was observed under a 125

dissecting microscope, and calanoid nauplii, cyclopoid copepods, poecilostomatoid copepods, large126

oceanic calanoid copepods, small calanoid copepods and other zooplankton taxa were sorted and127

counted. Calanoid copepods were identified to species according to Brodskii (1967), Frost (1974, 128

1989) and Miller (1988). Among calanoid copepods, large oceanic species, Neocalanus cristatus, 129

N. plumchrus, Eucalanus bungii and Metridia pacifica are known to account for 70% of the 130

mesozooplankton biomass in the subarctic North Pacific (Ikeda et al., 2008). For these species, 131

every copepodid stage (C1C6) was counted. In addition, female and male identifications were 132

made for C4C6 stages of E. bungii and M. pacifica. Eucalanus bungii nauplii, which 133

morphologically differ from other species and are easily identifiable (Johnson, 1937), were also 134

counted. Metridia pacifica performs diel vertical migration in the subarctic Pacific during summer 135

(Hattori, 1989; Padmavati et al., 2004; Yamaguchi et al., 2004; Takahashi et al., 2009). The C6 136

females of M. pacifica are more abundant near the surface at night than during the day, which 137

affects its apparent population structure and the zooplankton community structure. Saito et al.138

(2011) calculated the day: night ratio of M. pacifica C6F abundance in this region, and this ratio 139

was used to convert nighttime values to daytime values. For large copepods, the mean population140

stage was calculated using the following equation,141

7

6

1

6

1

ii

ii

N

NiMS ,142

where MS is the mean population stage, i is the copepodid stage (16), and Ni is the abundance (ind. 143

m2) of each stage (Marin, 1987). For E. bungii, whose nauplii were counted, a nauplius was 144

treated as stage 1, and MS was calculated using the value of each copepodid stage plus one, i.e. C1 145

was considered as 2. For C5 individuals of N. cristatus, N. plumchrus and E. bungii, the lipid 146

accumulation were observed and scored as three levels (1: no lipid, 2: some lipid, 3: full of lipid) 147

(Kobari and Ikeda, 1999, 2001; Tsuda et al., 1999, 2004; Shoden et al., 2005), and the mean lipid 148

score was calculated. The integrated mean temperature and phytoplankton fluorescence in the 149

0150 m profiles, total zooplankton abundance, the abundance and mean population stage of large 150

oceanic calanoid copepods and the mean lipid score of the C5 individuals were compared between 151

the sampling lines using Mann-Whitney U tests.152

Results153

Hydrography154

Based on the SLA data in the sampled area, the 5115N (western) line crossed an 155

anticyclonic eddy with an SLA of 1035 cm and a diameter of ca. 200 km (Fig. 2A). Along the 156

5040N (eastern) line, an anticyclonic eddy with an SLA of 1025 cm was observed north of the 157

westernmost station, but this line did not cross the eddy. We named the 5115N line crossing the 158

mesoscale anticylonic eddy “Eddy line” and the 5040N line “Non-eddy line”.159

The eddy observed along the Eddy line was first detected in mid-February 2010 south of 160

Attu Island (eddy centre: 5210N, 17220E) (Figs. 2B, C). This eddy gradually increased in 161

area (Fig. 2D) as it moved southeastward during the next five months and reached the sampling area 162

(near 5110N, 17250E) on 7 July 2010. The SLA near the eddy centre, representing the strength 163

of the eddy, continuously increased, and the area oscillated at one to two month periods overlain on 164

8

a general increase from ~7,000 to ~18,000 km2 (Fig. 2D). Fig. 3 (A) shows vertical profiles of 165

temperature along the Eddy line and the Non-eddy line. Between 171.35E ( solid black circles) 166

and 173.49E (open red triangles) along the Eddy line, a subsurface cold water mass (3.04.0C at 167

26.326.8) was observed at 80200 m. A somewhat warmer water mass (4.04.5C at 168

26.527.5) was also seen in this section at 200500 m depth. In contrast, this warm water mass 169

(4.04.5C) spreads from 50 m to 350 m between 174.00E and 174.64E. The 170

temperature-salinity relation also separated the water mass into cold and warm volumes between 171

26.2 and 26.6 (Fig. 3B). Unlike the Eddy line, subsurface cold or warmer water masses were 172

not observed along the Non-eddy line, and the water mass structure was mostly uniform along the 173

section (Fig. 3A, B). Fluorescence was higher along the Eddy line than the Non-eddy line, 174

particularly between 172.50E and 174.64E at 2550 m depth (Fig. 3C).175

The range of integrated mean temperature at 0150 m depth was 4.15.4C along the Eddy 176

line and 4.75.1C along the Non-eddy line (Fig. 4). There was no significant difference in the 177

integrated mean temperature (U test, p > 0.05), but the eddy centre was colder. The range of178

fluorescence at 0150 m was 57.979.4 mg m2 along the Eddy line and 45.566.5 mg m2 along 179

the Non-eddy line (Fig. 4). There was no statistically significant difference in fluorescence 180

between the lines (p > 0.05), but it was high near the eddy centre.181

Total zooplankton abundance and taxonomic accounts182

Total zooplankton abundance ranged from 1.0 105 to2.7 105 ind. m-2 (mean: 1.7 105183

ind. m2) along the Eddy line and 1.11.4 105 ind. m-2 (mean: 1.3 105 ind. m2) along the 184

Non-eddy line (Fig. 5A), not statistically different (U test, p > 0.05). Relative numerical185

abundances of some groups were different between the lines. Calanoid copepod nauplii (range: 186

3.329.3%, mean: 20.7%) and cyclopoid copepods (range: 20.560.4%, mean: 34.2%) were 187

abundant along the Eddy line, and cyclopoid copepods (range: 20.341.4%, mean: 28.1%) and large 188

oceanic calanoid copepods (Neocalanus, Eucalanus and Metridia spp.) (range: 13.338.9%, mean: 189

9

26.4%) were abundant along the Non-eddy line (Fig. 5B). The numerical abundance of calanoid 190

nauplii, cyclopoid copepods and poecilostomatoid copepods were 4.8 104, 0.7 104 and 2.9 104191

ind. m2 respectively along the Eddy line and 4.1 104, 0.8 104 and 2.3 104 ind. m2 192

respectively along the Non-eddy line, and these were not statistically different between these lines193

(U test, p > 0.05).194

Calanoid copepods195

In the zooplankton samples, 18 species of calanoid copepods belonging to 14 genera were 196

observed (Table I). Six coastal species (Acartia longiremis, Calanus marshallae and four species 197

of Pseudocalanus) and four large oceanic copepods were detected along both lines. Five 198

deep-sea species (Candacia columbiae, Microcalanus pygmaeus, Paraeuchaeta elongata, 199

Pleuromamma scutullata and Scolecithricella minor) were observed along both lines, but 200

Aetideopsis rostrata and Racovitzanus antarcticus were found only along the Eddy line, and 201

Aetideus armatus and Heterorhabdus tanneri were identified only along the Non-eddy line. 202

Comparing these calanoid copepod abundance between the lines, the abundances of A. longiremis, 203

P. minutus and R. antarcticus were significantly greater along the Eddy line than the Non-eddy line 204

(U test, p < 0.05, Table I).205

Large oceanic calanoid copepods206

Numerical abundance of the large oceanic calanoid copepod N. cristatus was significantly 207

greater along the Eddy line (range: 0.85.2 103 ind. m2, mean: 2.7 103 ind. m2) than along the 208

Non-eddy line (range: 0.91.7 103 ind. m2, mean: 1.2 103 ind. m2) (U test, p < 0.05, Fig. 6A, 209

Table I). Its mean stage was significantly higher along the Eddy line (mean: 3.2) than along the 210

Non-eddy line (mean: 2.2) (p < 0.05, Table II), and C5 individuals were relatively more numerous 211

along the Eddy line. In contrast, N. plumchrus abundance was significantly greater along the 212

Non-eddy line (range: 2.010.0 103 ind. m2, 4.9 103 ind. m2) than along the Eddy line (range: 213

10

0.44.9 103 ind. m2, mean: 2.8 103 ind. m2) (p < 0.05, Fig. 6B, Table I). There was no 214

significant difference between the lines in its mean stage. Eucalanus bungii abundance was also 215

significantly higher along the Eddy line (range: 5.813.2 103 ind. m2, mean: 8.0 103 ind. m2) 216

than along the Non-eddy line (range: 2.69.2 103 ind. m2, mean: 5.5 103 ind. m2) (p < 0.05, 217

Fig. 6C, Table I), but there was no significant difference in its mean stage. The abundance of M. 218

pacifica was significantly greater along the Eddy line (range: 8.040.6 103 ind. m2, mean: 24.1 219

103 ind. m2) than along the Non-eddy line (range: 6.739.3 103 ind. m2, mean: 17.1 103 ind. 220

m2) (p < 0.05, Fig. 6D, Table I), but there was no difference in its mean stage.221

The mean lipid scores of N. cristatus and N. plumchrus C5 individuals were significantly 222

higher along the Eddy line (N. cristatus: 2.1 0.4, N. plumchrus: 2.6 0.2) than along the 223

Non-eddy line (N. cristatus: 1.7 0.2, N. plumchrus: 2.3 0.1) (U test, p < 0.05, Fig. 7, Table II). 224

On the other hand, there was no difference in the mean lipid score of E. bungii C5 individuals.225

Discussion226

Influences of the Aleutian eddy on zooplankton community227

In the present study, the zooplankton community in and near a mesoscale anticyclonic 228

Aleutian eddy (along the Eddy line) comprised more large oceanic copepods, particularly N. 229

cristatus and E. bungii than coastal copepods (Table I, Fig. 6A, C). The eddy formation and 230

modification processes may have influenced this result. For example, a Haida eddy that was 231

formed on the continental shelf off British Columbia, Canada was reported to transport coastal 232

water to offshore areas (e.g. Whitney and Robert, 2002), and three coastal copepods, A. longiremis, 233

Calanus marshallae and P. mimus C4-C6 were abundant inside it (Mackas and Galbraith, 2002; 234

Mackas et al., 2005). In contrast, the Aleutian eddy that was sampled in the present study was 235

formed and propagated in the offshore water south of the Aleutian Islands (bottom depth of ca. 236

4000 m, cf. Fig. 2B). The water mass structure at time of the eddy’s formation and throughout its 237

transit and growth is not fully understood. Furthermore, another anticyclonic eddy was observed 238

11

adjacent to this eddy, and eddy-eddy interaction between the two might have occurred. Eddy-eddy 239

interaction can cause a sudden increase in SLA, and water inside an eddy can exchange with other 240

water masses (Ueno et al., 2012). Thus, it is uncertain whether the water mass injected at the 241

formation remained in the eddy during the sampling period. We found that the large oceanic 242

copepods N. cristatus and E. bungii were abundant within the eddy, suggesting that the eddy may 243

have been composed of offshore water during the sampling period rather than coastal water. 244

Unlike in the Gulf of Alaska and the eastern Bering Sea shelf, in the western Aleutian Islands, 245

coastal area (the depth less than 200 m) is much smaller and strictly limited around the islands (Fig. 246

1B). The Aleutian eddy could draw coastal water into it; however, its mass compared with 247

offshore water is presumably much smaller. Therefore, more oceanic copepods could be drawn 248

into the Aleutian eddy rather than coastal copepods.249

Biological productivity of the Aleutian eddy250

In the present study, the abundance of most species of large calanoid copepods were 251

significantly greater inside the eddy than outside (Table I). Inside the eddy, the mean lipid score252

of N. cristatus and N. plumchurus were significantly greater, and the mean stage of N. cristatus was 253

more advanced (Table II). The high abundances, lipid accumulations and advanced life stages of 254

large oceanic copepods suggest better survival and growth conditions to for large copepods inside255

the eddy than outside.256

The eastern subarctic North Pacific around the study area is known to be a high nutrient 257

and low chlorophyll (HNLC) region (Reid, 1962; Anderson et al., 1969), and iron is thought to be a 258

major liming factor for phytoplankton growth there (Boyd et al., 2004). Nevertheless, the 259

mesoscale Aleutian anticyclonic eddy observed in the present study had higher fluorescence than 260

outside the eddy, and thus substantial phytoplankton biomasses (Fig 3C, Fig. 4). Mesoscale 261

anticyclonic eddies are reported to increase the nutrient supply supporting productivity because262

eddy/wind interactions and submesoscale processes force upwelling to the surface of nutrient-rich 263

12

water (e.g. McGillicuddy et al., 2007; Mahadevan et al., 2008). In the present study, the eddy area 264

was increasing (Fig. 2D), so the influence of eddy/wind interactions might be weak. The Aleutian 265

eddy in the present study seems to have been influenced by colder water from the offshore region 266

on the western side and by warmer water from the Alaskan Stream on the eastern side (Fig. 3A). 267

Alaskan Stream eddies south of the eastern Aleutian Islands have been reported to cause the268

Alaskan Stream to meander to the south, and presumably carry nutrient/chlorophyll-rich water to 269

the south (Ueno et al., 2010). The nutrient-rich/warm water presumably enters from the eastern 270

sides of those eddies, and colder water flows in to them from the western sides. These 271

advections and mixing are hypothesized to result in high phytoplankton concentration inside those 272

eddies. Although the phytoplankton concentration history in the Aleutian anticyclonic eddy before 273

our observations from the T/S Oshoro-maru is not known due to lack of satellite surface 274

chlorophyll data since the study areas was mostly covered by clouds, the high phytoplankton 275

concentration observed in the eddy presumably resulted in the greater lipid accumulations of large 276

oceanic copepods (Fig. 7).277

The influence of phytoplankton concentration (the concentration of food) on the mass of 278

large oceanic copepods has been documented. For example, Dagg (1991) reported that in the 279

Bering Sea, where food was abundant, the carbon content in one N. plumchrus C5 individual was 280

416 g C ind. 1, whereas in an offshore region of the Gulf of Alaska, where food was scarce, the 281

carbon content was only 59143 g C ind. 1. In the present study, the relatively greater282

abundance and lipid accumulations in N. cristatus and N. plumchrus within the mesoscale 283

anticyclonic Aleutian eddy are thought to have resulted from stronger survival and growth rates 284

supported by greater food availability. That, in turn would have been generated by high primary 285

production enhanced by the advection of nutrient-rich water and cold water into the eddy.286

287

Conclusions288

The Aleutian eddy we studied was formed south of the Aleutian Islands, and some water 289

13

exchange due to eddyeddy interaction might have occurred after the initial formation. Since 290

large oceanic copepods were abundant during the sampling, the eddy was presumed to include a 291

substantial proportion of oceanic water. In addition, the high abundance and lipid accumulations292

of oceanic copepods and the advanced life stages in some species probably reflect high primary 293

production caused by the advective transfer into the eddy of colder nutrient-rich waters. In the 294

future, time-series analyses of the eddy modification process, primary production, phytoplankton 295

community and zooplankton community are required to more fully understand the effects of 296

Aleutian eddies on their entrained zooplankton communities.297

Acknowledgements298

We express our thanks to Associate Professor John Richard Bower, Marine Environmental 299

Science Laboratory, Faculty of Fisheries Sciences, Hokkaido University for his careful 300

proofreading of the English in a draft version of the manuscript prior to submission. We also wish 301

to acknowledge the captains, officers and crew members of T/S Oshoro-Maru, Faculty of Fisheries, 302

Hokkaido University and the members of the Plankton Laboratory, Faculty of Fisheries, Hokkaido 303

University for their help in sampling at sea. The altimeter products were produced by 304

SSALTO/DUCSCS and distributed by AVISO with support from CNES. We also thank Dr Roger 305

Harris, Editor in Chief, Dr Lulu Stader, Managing Editor, Associate Editor, two anonymous 306

reviewers of our manuscript. Their comments were helpful and greatly improved the present 307

manuscript.308

Funding309

The present study was partially supported by Grant-in-Aid for Young Scientists (B) 310

50344495 and (A) 24248032, 25257206/25121503 of the Japan Society for the Promotion of 311

Science (JSPS).312

14

References313

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Table and Figure legends440

Table I. The list of calanoid copepod species identified along the Eddy line (EL) and the 441

Non-eddy line (NEL) during 78 July 2010. Values are mean standard deviation of 442

abundance (ind. m2). Differences between two lines were tested by Mann-Whitney U test. 443

*: p < 0.05, NS: not significant.444

Table II. Comparison of mean stage and mean lipid score of large calanoid copepods between the 445

Eddy line (EL) and the Non-eddy line (NEL) during 78 July 2010. Differences between the 446

two lines were tested by Mann-Whitney U test. *: p < 0.05, NS: not significant.447

Fig. 1. The geographical distribution of mesoscale anticyclonic eddies along the Alaska Current 448

and the Alaskan Stream in the subarctic Pacific (A). A box indicates the study area magnified 449

in (B). Sampling stations along lines of mesoscale anticyclonic eddies during 78 July 2010 450

(B). Open and filled symbols in (B) indicate stations where XCTD and CTD casts were 451

conducted, respectively.452

Fig. 2. Sea level anomaly (cm) along the sampling lines on 7 July 2010 (A). Bathymetric 453

contours are also shown every 1000 m in (A). A trajectory of mesoscale anticyclonic eddy 454

from 10 February to 7 July 2010 in 7-day intervals (B). Diamond symbols in (B) indicate the 455

centre of the eddy in each time period, and filled symbols show the eddy’s origin and its456

position on 7 July 2010. Time series of position: latitude (filled circles) and longitude (open457

circles) (C), area (filled triangles) and sea level anomaly (open triangles) (D) of the mesoscale 458

anticyclonic eddy in seven-days intervals from 10 February to 7 July 2010.459

Fig. 3. (A) Temperature distribution (C as colour scale) superimposed by the density distribution 460

(, contours) for 01000 m depth, (B) temperature-salinity relation and (C) fluorescence 461

distributions for 0150 m along the Eddy line and Non-eddy line. Symbols along at the tops462

of (A) and (C) represent the locations of profiles characterized by T-S relations in (B).463

Fig. 4. 0150 m integrated mean temperature (filled circles) and fluorescence (open circles) along 464

the Eddy line and the Non-eddy line during 78 July 2010.465

20

Fig. 5. Total zooplankton abundance (A) and its taxonomic composition (B) along the Eddy line 466

and the Non-eddy line during 78 July 2010.467

Fig. 6. Abundance, stage composition and mean population stage of Neocalanus cristatus (A), N. 468

plumchrus (B), Eucalanus bungii (C) and Metridia pacifica (D) along the Eddy line and the 469

Non-eddy line during 78 July 2010.470

Fig. 7. Mean lipid scores of C5 individuals of Neocalanus cristatus, N. plumchrus and Eucalanus 471

bungii along the Eddy line and the Non-eddy line during 78 July 2010.472

Table I: The list of calanoid copepod species identified along the Eddy line and the Non-eddy line along 7-8 July 2010.

Functional group/SpeciesAbundance (ind. m2)

U testEddy line Non-eddy line

Coastal species

Acartia longiremis 902 180 367 106 EL > NEL*

Calanus marshallae 163 248 269 185 NS

Pseudocalanus mimus 2,792 1,227 2,288 1,063 NS

Pseudocalanus minutus 3,177 590 2,412 1,571 EL > NEL*

Pseudocalanus moultoni 1,431 408 1,463 1,074 NS

Pseudocalanus newmani 625 183 936 728 NSDeep sea species

Aetideopsis rostrata 15 34 0 NS

Aetideus armatus 0 92 142 NS

Candacia columbiae 15 34 451 903 NS

Microcalanus pygmaeus 13,391 4,166 6,773 4,171 NS

Paraeuchaeta elongata 148 113 69 138 NS

Pleuromamma scutullata 30 68 94 188 NS

Racovitzanus antarcticus 89 38 0 EL > NEL*

Scolecithricella minor 733 339 794 553 NSLarge oceanic species

Eucalanus bungii 7,973 3,010 5,492 2,741 EL > NEL*

Metridia pacifica 24,076 11,747 17,068 15,296 EL > NEL*

Neocalanus cristatus 2,709 1,645 1,175 352 EL > NEL*

Neocalanus plumchrus 2,790 1,874 4,930 3,694 NEL > EL*

Values are mean standard deviation of abundance (ind. m2) along the Eddy line (EL) and the Non-eddy line (NEL). Differences between the two lines were tested by Mann-Whitney U test. *: p < 0.05, NS: not significant.

Table II: Comparison of mean stage and mean lipid score of large calanoid copepods between the Eddy line (EL) and the Non-eddy line (NEL) during 78 July 2010.

Parameter/SpeciesMean sd.

U testEddy line Non-eddy line

Mean stage

Eucalanus bungii 3.9 0.2 3.8 0.4 NS

Metridia pacifica 2.5 0.4 2.6 1.2 NS

Neocalanus cristatus 3.2 0.7 2.3 0.3 EL > NEL*

Neocalanus plumchrus 4.3 0.2 4.3 0.6 NSMean lipid score Eucalanus bungii C5 2.2 0.2 2.1 0.2 NS Neocalanus cristatus C5 2.1 0.4 1.7 0.2 EL > NEL* Neocalanus plumchrus C5 2.6 0.2 2.3 0.1 EL > NEL*

Differences between the two lines were tested by Mann-Whitney U test. *: p < 0.05, NS: not significant.

54N

52N

50N168E 172E 176E 180 176W

Subarctic Pacific

160E 170E 180 170W 160W 150W 140W 130W 120W

60N

55N

50N

45N

(A)

(B)

Bering Sea

Subarctic PacificAleutian eddy

Yakutateddy

Sitkaeddy

Haidaeddy

Kenaieddy

Fig. 1. (Saito et al.)

Attu Island Bering Sea

36

24

12

0

172.5

(C)

Fig. 2. (Saito et al.)

Latit

ude

(N

)

0

7

14

(D)2151

51.5

52

52.5

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Are

a (

103

km2 )

173

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itude

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Feb. Mar. Apr. May June July2010

Sea

leve

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y (c

m)

171E 172E 173E 174E 175E 176E

53N

52N

51N

(B)

Attu Island

7 Apr.

5 May7 Jul.

10 Feb.3 Mar.

2 Jun.

52N

51N

50N

49N

100

5

0

10

(A)

170E 172E 174E 176E 178E 180E

10

Dep

th (m

)

177E 178E

172E 173E 174E 177E 178E

Salinity (psu)

Tem

pera

ture

(C

)

(B)

(A)

Dep

th (m

)

Eddy line Non-eddy line

172E 173E 174E

No data

No data

(C)

Fig. 3. (Saito et al.)

Temperature (C)

Fluorescence (mg m3)

0

200

400

600

800

1000

8

7

6

5

4

332.5 33.0 33.5 34.0 32.5 33.0 33.5 34.0

0

50

150

100

6

5

4

3170E 172E 174E 176E 178E 180E

80

70

60

50

40

Inte

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ed m

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tem

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ture

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150

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ence

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50 m

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Eddy line Non-eddy line

Fig. 4. (Saito et al.)

179E178E177E176E175E174E173E172E0

25

50

75

100

Calanoid naupliiCyclopoid copepods

Poecilostomatoid copepods

Large oceaniccalanoid copepods

Small calanoid copepods

Other zooplankton taxa

Zoop

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ton

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(Ｂ)

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1

2

3

Fig. 5. (Saito et al.)

0(D) Metridia pacifica

172E

0

25

50

75

100 6

6

4

2

0

(A) Neocalanus cristatus

Abun

danc

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ind.

m2

)

(B) Neocalanus plumchrus

(C) Eucalanus bungii

12

8

4

0100

75

50

25

0

15

10

5

0100

75

50

25

0

45

30

15

0100

75

50

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4

2

0

6

4

2

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4

2

6

4

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)

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tion

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Mea

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age

Mea

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age

Mea

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Eddy line Non-eddy line

Fig. 6. (Saito et al.)

179E178E177E175E 176E174E173E172E0

1

2

3

Eddy line Non-eddy line

Neocalanus cristatus C5Neocalanus plumchrus C5Eucalanus bungii C5

Mea

n lip

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core

Fig. 7. (Saito et al.)